This invention relates to a self-triggering imaging assembly for imaging radiation and to a self-triggerable imaging system.
Imaging devices comprising an array of image elements of various types are known
Charged coupled image sensors (also known as charged coupled devices (CCDs)) form one type of known imaging device. A CCD type device operates in the following way:
1. Charge is accumulated within a depletion region created by an applied voltage. For each pixel (image cell) The depletion region has a potential well shape and constrains electrons under an electrode gate to remain within the semiconductor substrate.
2. Voltage is applied as a pulse to the electrode gates of the CCD device to clock each charge package to an adjacent pixel cell. The charge remains inside the semiconductor substrate and is clocked through, pixel by pixel, to a common output.
During this process, additional charge cannot be accumulated.
Another type of imaging device which is known is a semiconductor pixel detector which comprises a semiconductor substrate with electrodes which apply depletion voltage to each pixel position and define a charge collection volume. Typically, simple buffer circuit read out the electric signals when a photon is photo-absorbed or when ionising radiation crosses the depletion zone of the substrate. Accordingly pixel detectors of this type typically operate in a pulse mode, the numbers of hits being accumulated externally to the imaging device. The buffer circuits can either be on the same substrate (EP-A-0,287,197) as the charge collection volumes, or on a separate substrate (EP-A-0,571,135) that is mechanically bonded to a substrate having the charge collection volumes in accordance with, for example, the well known bump-bonding technique.
A further type of device is described in International application WO95/33332. In WO95/33332, an Active-pixel Semiconductor Imaging Device (ASID) is described. The ASID comprises an array of image elements including a semiconductor substrate having an array of image element detectors and a further array of image element circuits. The image element detectors generate charge in response to instant radiation. Each image element circuit is associated with a respective image element detector and accumulates charge resulting from radiation incident on the image element detector. The image element circuits are individually addressable and comprise circuitry which enables charge to be accumulated from a plurality of successive radiation hits on the respective image element detectors. The device operates by accumulating charge on the gate, for example, of a transistor. Accordingly, analogue storage of the charge value is obtained. At a determined time, the charge from the image element circuits can be read out and used to generate an image based on the analogue charge values stored in each of the image element circuits.
CCD devices suffer from disadvantages of limited dynamic range, due to the limited capacity of the potential well inside the semiconductor substrate, and also to the inactive times during which an image is read out. Pulse counting semiconductive pixel devices also have the disadvantage of limited dynamic range. As these devices read the pixel contact when a hit is detected, they suffer from saturation problems at high counting rates. The semiconductor image element device according to WO95/33332 provides significant advantages over the earlier prior art by providing a large dynamic range for the accumulation of images.
It has been proposed to utilise the above-mentioned CCD and semiconductor devices to replace the film used in conventional radiation imaging systems, in order to provide real-time imaging and a more controlled lower dosage of radiation for a given exposure.
In a known arrangement, a CCD is electrically connected to an X-ray source. When the X-ray source is energised a start signal is transmitted alone the connecting wire to the CCD and its control circuitry to begin image acquisition and read-out.
In a optional arrangement disclosed in U.S. Pat. No. 5,513,252 there is no connection to the X-ray source. Instead, the CCD is continually read-out prior to radiation. A signal derived from the CCD is compared with a reference level. If the signal exceeds the reference level, the image acquisition of the CCD is initiated, that is to say the CCD stops being read out and the image starts to accumulate on the CCD.
European Patent Application Publication No. 0 756 416 A1 discloses a CCD used as an imaging device in which charge accumtulated in the CCD elements is clocked from several rows into a register in order to sum the charges. The summed result is put to a threshold test. Onset of X-ray radiation is detected when the signal applied to the threshold test exceeds a reference level. Image acquisition is then initiated, as described above i.e. only then will the CCD start accumulating the image.
In yet another arrangement the X-ray source and CCD have again no physical connection. A further sensor is arranged close to the imaging array for the CCD to detect the onset of X-ray radiation. On detection of incident X-ray energy, the sensor sends a signal to the CCD control circuitry to initiate image acquisition, as before.
The foregoing prior art systems involve a delay between activation of the radiation source and initiation of image acquisition. Since in radiation imaging, in particular X-ray imaging, the exposure to irradiation and radiation devices should be kept as low as possible it is desirable to reduce the delay as much as possible. Furthermore, the CCD approach is unsuitable for determining an end of an exposure. An additional sensor or a connection to the radiation source is necessary to provide an exposure trigger indicating end of irradiation.
In accordance with an embodiment according to a first aspect of the invention there is provided a semiconductor radiation imaging assembly, comprising: a semiconductor imaging device including at least one image element detector, said imaging device arranged to receive a bias for forming said image element detector; and bias monitoring means for monitoring said bias for determining radiation incident on said image element detector.
In accordance with an embodiment according to a second aspect of the invention, there is provided a method for providing a semiconductor imaging assembly, including an image element detector, comprising: monitoring a bias for said image element detector to determine radiation incident on said image element detector; and initiating a trigger for said bias fulfilling a predetermined condition.
In accordance with an embodiment according to a third aspect of the invention, there is provided a self-triggerable semiconductor radiation imaging system, comprising: a semiconductor imaging assembly as or operable as described in the foregoing paragraphs; control electronics coupled to said imaging assembly for receiving signals, including trigger signals, therefrom; signal storage means for storing signals coupled from said control electronics; an image processor for processing signals coupled from said control electronics; and a display unit for displaying images provided by said image processor.
Embodiments in accordance with the first, second or third aspects of the invention advantageously provide a substantially instantaneous or real-time response to radiation incident on an or a plurality of image element detectors by monitoring the bias applied to form the image element detector/s. Such embodiments may provide trigger signals in direct response to radiation incident on the image element detector/s, yet by indirect monitoring of the incident radiation, thereby obviating the need for reading out data from the image elements. Further, the embodiments provide for self-triggering image detector devices and systems, and obviate the need for trigger signals to be provided from X-ray sources to the control electronics of such systems for identifying beginning and/or ending of exposure.
Furthermore, since the bias represents an average of the radiation incident over the whole area of an array of image elements it provides a robust indication of the total radiation incident over that area, and provides a sensitive self-triggering mechanism.
Suitably, the semiconductor imaging device comprises a semiconductor substrate supporting a first and second conductive layer on respective first and second surfaces. The first and second conductive layers at least partially oppose each other for applying the bias between them to form a radiation detection zone for the image element detector.
Typically, the first conductive layer comprises a substantially continuous layer across the first substrate surface, and the second conductive layer comprises a plurality of image element electrodes for defining respective radiation detection zones for a plurality of image element detectors.
Advantageously, the bias monitoring means is adapted to provide a trigger for the bias fulfilling a predetermined criterion.
In accordance with a first preferred embodiment, the bias monitoring means determines a rate or direction of change of the bias, and more preferably discriminates between different rates or direction of change. Thus, increases and decreases in bias due to corresponding increases and decreases in incident radiation intensity may be determined and utilised to initiate suitable trigger signals.
Preferably, one or more threshold values are set corresponding to bias levels representative of incident radiation levels at start of exposure and/or end of exposure of which triggers are to be initiated. Such triggers are initiated for the bias transgressing respective bias levels at and/or in an appropriate direction.
The bias monitoring means preferably monitors the bias current, and provides a signal representative of the bias current, although for an assembly having the bias supplied from a non-constant output voltage supply the bias voltage may be monitored. This representative signal is differentiated, the resulting signal low-pass filtered (by an integrator for example) and the filter result input to a comparator for comparing with one or more threshold levels. Thus, typically the threshold levels are compared against a value of an intermediate signal representing the incident radiation and derived from the bias current.
An example of a suitable differentiator is a high pass filter, and an example of a suitable low pass filter is an integrator. Cut-off frequencies for the high pass filter differentiator and the low pass filter integrator suitably lie in the ranges 10-200 Hz. and 500 Hz.-2 kHz. respectively.
In accordance with a second preferred embodiment of the invention, the bias monitoring means accumulates a bias value which represent aggregate radiation incident on the image element detector/s, which advantageously offers improved radiation detection rejection against reliability. Thus, the likelihood of false positive triggers is reduced without adversely affecting the sensitivity to incident radiation.
Preferably, an image element dark or quiescent bias value is subtracted from a value representing the bias, and the resulting signal is then integrated to provide an indication of the total accumulated bias value after correction for the quiescent bias value. One or more threshold levels arc set against which the accumulated bias value is compared, to initiate suitable trigger signals such as start of and/or end of exposure trigger signals. Preferably, for an imaging device comprising more than one image element detector, then a dark or quiescent bias value corresponding to all of the individual image elements is subtracted from the value representing the bias value.
Suitably, sample and hold circuitry is configured to record the bias value prior to a radiation exposure in order to obtain a suitable quiescent bias value.
In accordance with a third preferred embodiment of the invention, an integrated signal representative of the bias, preferably bias current but optionally voltage, is subtracted from the signal representative of the bias to derive a signal corresponding to the radiation incident on the image element detector/s. The resulting signal is then compared with threshold values to initiate suitable trigger signals.